Polyimide has superior heat resistance and electrical insulation and is mainly used as a polyimide film in applications such as flexible printed circuit boards, etc. The requirements of polyimide films used in these applications have increased with miniaturization of the flexible printed circuit boards and semiconductor packages. For example, to have a linear thermal expansion coefficient comparable to metals, a high elastic modulus, and minimal dimensional change from water absorption are necessary properties for the polyimide films and polyimide films that meet these requirements have been developed (Patent Documents 1-6).
Examples of a three-component polyimide consisting of anhydrous pyromellitic acid, 4,4′-diaminodiphenyl ether, and paraphenylene diamine, which combined the paraphenylene diamine as the diamine compound for enhancing the elastic modulus are described in Patent Documents 1-6. Furthermore, development to four-component polyimide has also been carried out, which added 3,3′-4,4′-biphenyl tetracarboxylic dianhydride to the aforementioned three-component polyimide to further enhance the elastic modulus (Patent Documents 7, 8). In addition, an attempt has been made to improve the properties by controlling the procedure for adding the monomer at the time of polymerization in the four-component polyimide (Patent Document 9). An attempt has also been made to improve the properties by carrying out drawing at the time of the film is fabricated (Patent Documents 10).
Furthermore, development of a polyimide film, which provided anisotropic property by setting the linear thermal expansion coefficient in the machine direction (hereinafter also referred to as MD) of the film to be smaller than the linear thermal expansion coefficient in the transverse direction (hereinafter also referred to as TD) of the film in order to suppress the dimensional change during the process of bonding with the metal has also been carried out (Patent Documents 11). The purpose of this is to offset the phenomenon wherein elongation occurs in MD of the film and contraction occurs in TD of the film from tension being applied in the FPC (Flexible Printed Circuit) process wherein the lamination method is usually employed and carries out the bonding with the metal by roll-to-roll heating.
However, in all of the aforementioned polyimide films, difference was created in the orientation angles at the center part and the end parts in the transverse direction in polyimide films obtained according to drawing, and variance was created according to the position on the film. Consequently, problems such as the physical properties varying according to the using section were created, and there was a need to develop a polyimide film having uniform orientation angles in the transverse direction of the film.